Recent research and development efforts on a display apparatus have been directed to reduction in a thickness of the display apparatus and energy saving. For example, there are liquid crystal display, plasma display, electric field emission type display, and the like.
So-called field emission display (hereinafter, referred to as FED for short) may be of a spindt type having a vertical configuration, or an edge emitter type having a horizontal configuration, which is referred to as a lateral type, a side type, or a planar type, depending on an array of a cathode electrode, a gate electrode, and an anode electrode.
A spindt type FED includes a substrate, a cathode electrode (emitter electrode) of an electron emission unit formed thereon having a substantially conical shape, and a gate electrode of a lead-out electrode stacked on a substrate around the cathode electrode having an insulating layer therebetween. In the spindt type FED, a voltage is applied between the cathode electrode and the gate electrode in a vacuum to thereby produce a high electric field therebetween. As a result, electrons are emitted from a tip end of the cathode electrode through the electron emission mechanism in an electric field.
In the configuration of the spindt type FED, a distance between the emitter electrode and the gate electrode is determined by a hole size provided in a resist pattern so that it is necessary to enhance accuracy in a lithography and an etching process in order to reproducibly and uniformly form the emitter electrode of an element for emitting a plurality of electrons. However, applications of these technologies largely depend on the performance of an apparatus, which cannot be controlled easily. In other words, there are inevitable problems on the fabrication caused due to miniaturization in which an electron emission characteristic varies for each element depending on the shape of the emitter electrode and the distance between the emitter electrode and the gate electrode. In particular, when producing a large-screen FED, it is difficult to uniformly form the emitter electrode on a large substrate. Therefore, unless the array of the emitter electrode is formed uniformly, the field electron emission characteristic varies depending on a position on the screen, thereby making it difficult to have a good image display on the screen.
On the other hand, the edge emitter type FED is configured such that field electrons are emitted from an emitter electrode to an anode electrode, wherein the emitter electrode is formed above a gate electrode via an insulating layer in a substantially flat sheet shape and an electric field is generated between the gate electrode and the emitter electrode to thereby cause the emitter electrode to emit electrons.
In an edge emitter type electron emission apparatus having the aforementioned configuration, electrons emitted from the emitter electrode are accelerated to collide with a fluorescent body in the same way as in the spindt type electron emission apparatus. Thus, in the FED employing the edge emitter type electron emission apparatus, the fluorescent body is excited to emit light to thereby make an image be displayed.
In such an edge emitter type FED, the edge emitter electrode for emitting electrons can be formed substantially in a flat sheet shape, and the electrons are emitted by an electric field generated between the gate electrode and the emitter electrode. Accordingly, this type of electron emission apparatus can be easily produced compared with the aforementioned spindt type FED.
The edge emitter type FED having such characteristics is applied to a field of a planar type multipole vacuum tube, and may be applied in a flat panel display by being plurally arrayed on a plane. Further, the edge emitter type FED has superior responsiveness, brightness, environment resistance, and the like, as compared with a liquid crystal display. Thus, there is a possibility for the edge emitter type FED becoming a main trend in the flat panel display, so that active research and development on a large scale planar type display apparatus have been in progress.
For resolving the aforementioned problems of the spindt type FED, Japanese patent Laid-Open Publication No. 1991-295131 (hereinafter, referred to as ‘reference 1’) discloses an edge emitter type field emission display element for use in a light emitting type display apparatus and the like as shown in FIG. 8, in which a lateral type emitter is thinly formed.
FIG. 8A describes a cross sectional view of a prior art field emission display element disclosed in reference 1, and FIGS. 8B to 8F show a manufacturing method thereof.
A field emission display element 100 includes an insulating plane substrate 101; pedestals 102 and 102′ made of a silicon dioxide thin film and formed on a surface of the plane substrate; a cathode electrode 103 of a conductive thin film formed on the pedestals and having cathode tip 104 whose end portion is formed in an acute shape, i.e., a saw-toothed shape and from which electrons are emitted; an anode electrode 105 formed above the surface of the plane substrate and facing a cathode substrate; and a gate electrode 106 formed above the surface of the plane substrate and self-aligned with the cathode tip 104 in the cathode electrode.
As shown in FIGS. 8B to 8F, the manufacturing method includes the steps of;
forming an insulating thin film 107 made of, e.g., a insulative silicon dioxide on a surface of a plane substrate 101 (FIG. 8B);
forming a pedestal-shaped resist pattern 108 on the surface of the insulating thin film 107 (FIG. 8C);
etching the insulating thin film 107 to have a reverse tapered shape by using the resist pattern 108 as a mask, and thus, forming pedestals 102 and 102′ (FIG. 8D);
forming an aluminum thin film 109 over the surface of the plane substrate by employing a directional particle beam method after removing the resist (FIG. 8E); and
etching the conductive thin film 109 to form a cathode electrode 103, a gate electrode 106, and an anode electrode 105 by using a photo process (FIG. 8F).
In the prior art disclosed in reference 1, a distance between the cathode electrode and the gate electrode is controlled by a thin film thickness, so that uniform electrical characteristics can be obtained over large area. Further, it is possible to reduce the distance between both electrodes and to form the end portion of the cathode tip 104 in an acute shape, whereby a gate threshold voltage can be lowered. Meanwhile, in the aforementioned prior art, improvements have been achieved by further sharpening the cathode electrode (edge emitter electrode) and making a gap between the cathode electrode and the gate electrode small, thereby lowering a driving voltage.
Japanese patent No. 2613697 (hereinafter, referred to as ‘reference 2’) discloses a field emission display element having a collector electrode (anode electrode) and an edge emitter electrode (cathode electrode) in the same plane, similarly to the field emission display element disclosed in reference 1. Further, the aforementioned prior art suggests display apparatus as an electron source for display apparatus, which has a configuration that a transparent anode electrode is formed on a substrate, and a fluorescent body formed on the anode electrode is excited to emit light.
Japanese patent No. 2635879 (hereinafter, referred to as ‘reference 3’) discloses a planar type (edge emitter type) FED shown in FIG. 9, in contrast with the aforementioned prior arts having an edge emitter electrode and a gate electrode in the same plane.
An electron emission element 341 includes an emitter 314 and a gate 316 of a stacked structure, and an anode 343 made of a conductive thin film stacked on a transparent substrate 342. In the anode 343, there is stacked a fluorescent body 344 for slow electron beams, and the transparent substrate 342 is separated from another substrate 312 by a proper distance while facing each other. In this substrate 312, there are stacked the emitter 314 and the gate 316. In addition, the fluorescent body 344 on the transparent substrate 342 is arranged to face the gate 316.
In the electron emission element 341, a voltage is applied between the emitter 314 and the gate 316 to emit electrons and a higher voltage is applied between the emitter 314 and the anode 343 such that the electrons emitted between the emitter 314 and the gate 316 are attracted toward the anode 343 side, as indicated by an arrow A in FIG. 9. These electrons collide with the fluorescent body 344 and emit light just before reaching the anode 343.
Since the gate electrode can be stacked on the edge emitter electrode via an insulator, an edge of the edge emitter electrode can be drawn close to that of the gate electrode and an electric field can be applied efficiently to the edge of the edge emitter electrode. Further, by sharpening the edge of the edge emitter electrode, it is possible to efficiently focus an electric field from the edge emitter electrode to the sharpened tip end.
By plurally arraying such an electron emission element 341 as one pixel, a flat panel display apparatus can be obtained. In this kind of flat panel display apparatus, if the distance between the electron emission elements 341 is slightly greater than that between the emitter 314 and the gate 316, the electron emission element 314 does not affect a neighboring one, even when the electron emission element 341 forming each pixel is drawn close to the neighboring one. Further, even if pixels are closely placed each other by making a gap between the pixels small and a plurality of wires perpendicular to the transparent electrode 342 side and another substrate 312 side are formed, there is no problem such as crosstalk or the like. Therefore, it is possible to adopt a simple matrix mode as a driving mode.
Meanwhile, in a conventional planar type FED, it is difficult to deflect electrons emitted from the edge emitter electrode to a desired direction and to apply the planar type FED to a practical FED. For this, Japanese Patent Laid-Open Publication No. 1999-232997 discloses (hereinafter, referred to as ‘reference 4’) a four-layered electron emission element as shown in FIGS. 10 and 11, including a pair of gate electrodes; an emitter formed between the gate electrodes via insulating layers; and an auxiliary electrode formed on a bottom surface. In the four-layered FED of reference 4, an electric field generated from the auxiliary electrode does not affect the emitter electrode, whereby such a problem can be solved that the electric fields applied to the emitter electrode from the gate electrodes become relatively lowered. Further, it is possible to deflect the electrons emitted from the emitter electrode to a desired direction, and, at the same time, to have electrons emitted efficiently even at a low driving voltage.
In FIGS. 10 and 11, FED 401 includes a supporter 402, and a face plate 404 on which anode electrodes 403 are formed of a stripe shape. In the face plate 404, there are formed a red fluorescent body 405R, a green fluorescent body 405G, and a blue fluorescent body 405B, each of a rectangular shape and emitting light on the anode electrode 403. By these three colors of the fluorescent bodies, one pixel is formed of a substantially square shape (hereinafter, each fluorescent body is referred to as a sub-pixel, and an area where three colors of the fluorescent bodies are assembled is referred to as a pixel.
The FED 401 is formed on an insulating substrate 406 and arranged in a matrix form, and has a predetermined layered structure. Further, the FED 401 has an opening hole (well) 407 formed in a stacked direction of the layered structure and is of a substantially rectangular shape. From the opening hole 407, electrons are emitted.
As shown in FIG. 11, the FED 401 in the four-layered electron emission element has the insulating substrate 406 such as a glass; an auxiliary electrode 411 formed on the insulating substrate 406; a first gate electrode 413 stacked on the auxiliary electrode 411 via a first insulating layer 412; an edge emitter electrode 415 stacked on the first gate electrode 413 through a second insulating layer 414; and a second gate electrode 417 stacked on the edge emitter electrode 415 via a third insulating layer 416.
In the FED 401, the opening hole 407 goes through the first insulating layer 412, the first gate electrode 413, the second insulating layer 414, the edge emitter electrode 415, the third insulating layer 416, and the second gate electrode 417, and, at the same time, is formed to expose the auxiliary electrode 411 to a bottom surface. Further, in the FED 401, the first gate electrode 413 is configured to protrude from an opening edge of the edge emitter electrode 415 into an inner side.
By applying a predetermined voltage to the first electrode 413 and the second gate electrode 417, an electric field is generated between the first gate electrode 413, the second gate electrode 417, and the edge emitter electrode 415. Further, through the field electron emission mechanism, electrons are emitted from the tip end of the edge emitter electrode 415 in the direction of the anode electrode 405, i.e., substantially perpendicular to in-surface of the auxiliary electrode 411, and the electrons emitted from the edge emitter electrode 415 are deflected to a direction of the anode electrode 403.
Accordingly, in the FED 401, the electrons emitted from the edge emitter electrode 415 can be made to collide efficiently with the fluorescent body 405 formed on the anode electrode 403, whereby the fluorescent body 405 can efficiently emit light. Therefore, brightness of the FED can be markedly improved.
However, in the structure disclosed in reference 1, since the cathode electrode and the anode electrode are formed in the same substrate, a part corresponding to the cathode electrode cannot be a display unit. Therefore, it is difficult to be used in a high density display.
Further, in the structure disclosed in reference 3, electrons are emitted only from the tip end of the emitter and light emission becomes of a point shape such that it is difficult to uniformly emit light over the surface of a predetermined pixel. Still further, in the structure disclosed in reference 4, it is possible to improve focusing property, but there encounters a difficulty to commercialize a midsize display element due to a complicated structure thereof.
Thus, an emitter FED of a simplified structure has been studied as follows.
An edge emitter type FED is used for a midsize display element (20 to 30 inch diagonal screen), such as a television set for home use, or a display device for a personal computer having a pixel pitch in a range from about 0.6 to 1.0 mm, as mentioned in prior arts disclosed in Japanese Patent No. 2635879 and Japanese Patent Laid-Open Publication No. 1999-232997.
In the edge emitter type FED used for a midsize display element, a length direction of each sub-pixel is arranged to be parallel with that of an electron emission unit formed with an edge emitter electrode, assuming that the supporter 402 and the face plate 404 face each other, as shown in FIG. 10.
FIGS. 12A and 12B are plane view for exemplary electron emission element of a prior art developed for a midsize display device, and describe a relation of positions between fluorescent bodies 56R, 56G, and 56B, a cathode electrode 62, a gate electrode 61, and a well 59. Here, FIG. 12A is for a case having three wells and FIG. 12B is for a case having one well. Further, FIG. 13 shows a cross sectional view for a part of the electron emission element shown in FIG. 12A.
The electron emission element includes a gate electrode 52 formed on a substrate that is not shown; an insulating layer 53; a cathode electrode 54 (hereinafter, referred to as ‘edge emitter electrode’); and an insulating passivation that is not shown. In addition, a fluorescent body 56 and an anode electrode 57 are disposed in an upper part of the electron emission element.
The edge emitter electrode 54 formed in a substantially planar type, in particular, an edge 58 of a tip end thereof, which emits electrons, is configured to face the gate electrode 52 of a lower side via an insulating layer 53. The edge emitter electrode 54 forms a closed rectangular shape, and a well 59 for electron emission is made of a space surrounded by the edge emitter electrode 54 and a part where the insulating layer 53 directly below the edge 58 is removed (i.e., the insulating layer is etched such that a lower part of the edge 58 in the edge emitter electrode 54 protrudes from the insulating layer 53 to thereby face the gate electrode 52).
Respective potentials of gate electrode 52 and the edge emitter electrode 54 are fed by the gate feeder unit 61 and the cathode feeder unit 62, respectively. The gate electrode 52 is disposed to be perpendicular to the edge emitter electrode 54, and the edge emitter electrode 54 forming the well 59 corresponds to one sub-pixel 56R, 56G, or 56B, having three wells as one group, in case of FIG. 12A. Further, each pixel 60 includes three sub-pixels of a red fluorescent body 56R, a green fluorescent body 56G, and a blue fluorescent body 56B as one group, and a display for displaying full colors is achieved by forming the substantially square-shaped pixel 60 in a matrix.
The electron emission element serves as a display device in such a manner that a scan signal and a data signal are inputted to the gate electrode 52 and the cathode electrode 54 from the gate feeder unit 61 and the cathode feeder unit 62, respectively, and a predetermined pixel is selected and driven to thereby have light emitted.
Further, by an electric field generated between the gate electrode 52 and the edge emitter electrode 54, electrons are emitted from the edge emitter electrode 54, and the emitted electrons are accelerated by an electric field between the gate electrode and the anode electrode to collide with the fluorescent bodies 56R, 56G, and 56B, resulting in exciting the fluorescent bodies to emit light. Meanwhile, the insulating passivation that is not shown and is formed on the edge emitter electrode 54 is for keeping an insulation with the anode electrode 57.
In the electron emission element having such a configuration, a voltage is applied between the edge emitter electrode 54 and the gate electrode 52 to thereby emit electrons. Further, when a higher voltage is applied between the edge emitter electrode 54 and the anode electrode, the emitted electrons between the edge emitter electrode 54 and the gate electrode 52 are attracted toward the anode side, as indicated by arrows in FIG. 13. These electrons collide with the fluorescent bodies 56R, 56G, and 56B to thereby emit light right before reaching the anode.
By disposing such an electron emission element to each of the red fluorescent body 56R, the green fluorescent body 56G, and the blue fluorescent body 56B, a fluorescent body unit 56 including three fluorescent bodies 56R, 56G and 56B is formed, which functions as a pixel 60. Further, a flat panel display apparatus can be formed by arraying a multiplicity of the pixels 60.
Meanwhile, in the edge emitter type FEDs shown in FIGS. 12A and 12B, and 13, electrons are emitted from the electron emission unit (edge) 58 of the ridge-shaped cathode tip end in the edge emitter electrode 54, and an amount of electrons emitted is generally determined by work function, field intensity, and electron emitting area of the edge 58. Here, the work function is determined by a material of the emitter, which is practically limited to Mo, W, C, or the like, and therefore the value of the work function is substantially fixed.
Field intensity is limited in practical use due to limitations on a withstand voltage between the cathode and the gate, a driving withstand voltage of a driver, or the like. Therefore, electron emission capacity of the edge emitter electrode 54 is determined mainly on the basis of a ridge-shaped edge length (peripheral length of the well 59) in practical use.
However, in the configurations of the prior art edge emitter electrode 54 shown in FIGS. 12A and 12B, and 13, for example, in case where there is only one well 59 corresponding to each sub-pixel in a central portion, as shown in FIG. 12B, the length of the edge 58 of the edge emitter electrode 54 in each electron emission element is short for areas of the fluorescent bodies 56R, 56B, and 56G of each pixel 60. As a result, it is difficult to have a sufficient amount of electrons emitted even for a pulse width of about Du= 1/240 that is used in a graphic display, and difficult to achieve enough emission brightness even when an anode voltage applied ranges from 2 kV to 5 kV.
Meanwhile, it has been tried that a plurality of lines of wells 59 are arranged in parallel along a length direction (three lines of wells 59 are placed in each of the fluorescent bodies 56R, 56G, and 56B, in FIG. 12A) to thereby enlarge an electron emitting area, as shown in FIG. 12A.
Since the edge length along the longer side of the fluorescent body 56R, 56G, or 56B is longer than the shorter side, a large amount of electrons are emitted from the longer side, so that the longer side edge affects the surroundings significantly more than the shorter side edge.
Further, the electrons from the long edge are emitted along a direction normal to the longer side of the fluorescent body 56R, 56G, or 56B, and the electrons from the short edge are emitted along a direction parallel with the longer side of the fluorescent body 56R, 56G, or 56B.
At this time, as shown in FIG. 13, the gate electrode 52 (+) and the anode electrode (+) are placed under and above the edge emitter electrode 54 (−), and an electron emitted from the edge emitter electrode 54 is accelerated in a horizontal direction as well under the combined influence of two plus potentials. As a result, the angular spread of the electrons emitted ranges mostly within 60 degrees with respect to the cathode plane as shown in FIG. 13.
Here, since the distance from the edge 58 of the well 59 in the right side of the edge emitter electrode 54 corresponding to the fluorescent body 56R to the right side fluorescent body 56G is short and the directions of the electrons emitted are spread up to 60 degrees, electrons emitted from the well 59 are spread to be irradiated to the fluorescent body 56G as well. As a result, light corresponding to a color other than a selected one may be emitted, resulting in color mixing disorder, which is a fatal problem in a full-color display device.
Further, when the electron emission unit is made small by arranging only one well 59 in the cathode electrode as shown in FIG. 12B, the distance from the edge of the left side of the well in fluorescent body 56R to the neighboring fluorescent body 56G becomes large, as compared with the case of arranging three wells in the cathode electrode, whereby the color mixing disorder can be prevented. But, the amount of electrons emitted becomes reduced since the number of electron sources is decreased, so that high brightness cannot be obtained.
As described above, in the edge emitter type FED of a simple configuration without having a focusing electrode and without performing an anode selection, the development of a field electron emission apparatus that takes an electron path into consideration has not been successful.